1. Technical Field
The present invention relates to a stress sensitive element, in particular, to one suitably used for detection of stress such as acceleration by utilizing change in resonance frequency of a piezoelectric resonator.
2. Related Art
Acceleration sensors have been widely used not only in automobiles, airplanes and rockets but also in various industrial plants for the use of, for example, monitoring abnormal vibration. JP-A-2-248866 discloses a beam structure of an acceleration sensor shown in a perspective view of FIG. 6. In FIG. 6, the reference symbol 50 denotes a double-ended tuning fork type quartz resonator having two vibrating beams 51, the reference symbol 52 denotes a bonding portion of the double-ended tuning fork type quartz resonator 50, and the reference symbol 60 denotes a beam constituted by quartz having the same cutting angle as that of the double-ended tuning fork type quartz resonator 50. The beam 60 has projecting portions 70 having a thicker thickness than that of other parts thereof. The projecting portions 70 and the bonding portion 52 of the double-ended tuning fork type quartz resonator 50 are bonded by using adhesive or the like. Then, a weight 80 is provided on a free end of the beam 60 and the other end opposite to the weight 80 is fixed to a base 90.
The base 90 of the acceleration sensor constructed as shown in FIG. 6 is fixed on an object to be measured. When the acceleration is applied in the direction of the arrow, the weight 80 makes the beam 60 bend, and compression stress or extension stress is then applied to the double-ended tuning fork type quartz resonator 50 fixed to the beam 60, resulting in change of the frequency. In sum, the acceleration sensor is one that determines the magnitude of acceleration from the amount of the frequency change. According to the disclosure of JP-A-2-248866, the projecting portions 70 are formed on the beam 60, so the magnitude of stress applied to the double-ended tuning fork type quartz resonator 50 is increased as compared with the case in which no projecting portions, thereby making it possible to provide an acceleration sensor of high sensitivity while preventing the plate thickness of the beam 60 from becoming thinner and avoiding increase in the mass of the weight 80.
However, the acceleration sensor disclosed in JP-A-2-248866 is constructed in a way that the beam 60 constructed by quartz having the same cutting angle as that of the double-ended tuning fork type quartz resonator 50 is bonded to the resonator and the weight is bonded to the free end of the beam 60. As a result, there arises a problem in that the number of man-hours increases and the manufacturing cost becomes high.